Joint structure of metallic members, such as various joints, is produced by joining dissimilar metallic members. In the joining of dissimilar metallic members, brazing is conducted by irradiating a brazing filler metal interposed between those dissimilar metallic members with laser beam, and heating the brazing filler metal. A joining layer is formed thereby between the dissimilar metallic members. Thus, a joint structure of metallic members is produced.
For example, in the case of using a Fe-based metallic member containing a Fe-based material and an Al-based metallic member containing an Al-based material as dissimilar metallic members, Al and Zn do not form a compound layer and forms an eutectic structure in a wide range. For this reason, a Zn-based brazing filler metal is used as a brazing filler metal. This can ensure strength between the Al-based metallic member and the joining layer.
In order to suppress growth of a reaction layer (for example, intermetallic compound layer) formed in the interface part between the Fe-based metallic member and the joining layer, it is effective to decrease a reaction temperature and shorten a reaction time. For this reason, Al which decreases a melting point of a brazing filler metal by forming an eutectic alloy together with Zn is used as an additive element (for example, see Patent Document 1).
However, in the case that an intermetallic compound layer formed in the interface part between the Fe-based metallic member and the joining layer is brittle, the breakage may be generated in such the intermetallic compound layer. As a result, strength of a joint structure of dissimilar metallic members was considerably decreased as compared with that of a joint structure of similar metallic members.
When joining is conducted by laser irradiation at low temperatures, heat input into a joined part of dissimilar metallic members is conducted by thermal conduction from a surface of the brazing filler metal. Therefore, in the interface part of the joined part, thermal history is different every site. For this reason, in the interface part of the joined part, a reaction layer grows heterogeneously, thereby partially forming an unreacted layer and increasing a thickness of the reaction layer. As a result, joint strength was decreased. Particularly, in order to obtain good joining in the joining of dissimilar metallic members, a range of a joining temperature is limited to a predetermined range, unlike similar metallic members. Therefore, the problem due to the thermal history was serious.